The present invention relates generally to the field of computed tomography imaging systems. In particular, the invention relates to methods and systems for scatter control and reduction in a stationary computed tomography (SCT) system.
Computed tomography (CT) imaging systems have been developed over the past decades and now are prolific in medical diagnostics and other contexts. In general, such systems typically include an X-ray source, such as a conventional X-ray tube, positioned diametrically opposed to an X-ray detector. In a so-called third generation CT system design, the source and detector rotate on a gantry, and the source is triggered repeatedly or is on continuously during rotation to produce beams of X-ray radiation that are directed through a subject of interest and fall onto the detector on the opposite side of a gantry. Features and structures of the subject attenuate the emitted radiation, and the detector measures the transmitted radiation. The measurements are usually converted to attenuation measurements and the resulting measurement or projection data are then processed for reconstruction of useful images, typically presented as slices or images through the subject. Many such images may be produced in a single imaging sequence.
Next generation CT architectures, which include stationary computed tomography (SCT) concepts, offer high effective scan speeds by using distributed, stationary, addressable electron emitters to rapidly trigger electron beams onto a stationary, distributed X-ray target, thereby producing X-rays. The X-rays after traversing a subject of interest are collected by a stationary, distributed X-ray detector. SCT systems offer various advantages over the conventional CT systems.
Detectors for conventional CT systems using rotating gantries are becoming increasingly larger with small detector cells for improved imaging resolution, utilizing multiple rows to obtain significant axial coverage on the object of interest during an exam or scan. Circuitry associated with the detectors must also be rotated to perform the data acquisition and initial processing. Thus having a stationary detector and/or a stationary source in a SCT design lightens rotational loads, or even eliminates the need for rotation of system components all together. These systems are useful for generating high-quality images while reducing the mechanical, electrical, and thermal challenges associated with rotation of a source and a detector in a conventional CT system.
Though current CT imaging systems and SCT designs are very useful in identifying features of interest within a subject, they pose certain limitations. When a CT or SCT imaging system is operated, the passage of X-rays through an object results in a combination of photoelectric absorption, coherent scattering, and Compton scattering. In the CT and SCT imaging systems, it is the pattern of transmitted X-rays through the object that is useful for reconstructing an image of the object's interior structure. The scatter component degrades image quality in resulting reconstructions. Scatter in the measured data results in image artifacts and increases noise in the measurements, even if the scatter is accurately estimated and eliminated through special correction algorithms. Scatter artifacts can be mitigated by subtracting an estimate of the scatter, but the signal to noise ratio (SNR) cannot be recovered by scatter correction.
The X-ray beam used with most CT imaging systems for large volume coverage has a large cone angle; therefore, the amount of scattered radiation in the measurements increases. In conventional CT systems utilizing rotating gantries, an anti-scatter grid, sometimes referred to as a collimator, is placed in front of the detector, which selectively attenuates scattered X-rays but allows primary X-rays to reach the detector. Although the collimator plates are designed to have little effect on the transmission of the primary X-ray beam, they do attenuate it to some extent due to their finite cross-section. As individual CT detectors sizes get smaller, a larger fraction of the area of each pixel is shadowed by the collimator blade, leading to poor dose utilization—a smaller amount of dose applied to the patient is detected and used for diagnostic imaging purposes. The drawback of reducing the active area of the X-ray detector as sizes get smaller (since the collimator blades cover a larger portion of the active area) has been addressed primarily by making thinner collimator plates, which poses significant manufacturing and reliability challenges. There is a physical limit to how thin the collimator blades can be manufactured while still maintaining their utility.
So-called fourth generation CT scanners use a fixed array of detectors and a rotating X-ray source. SCT imaging systems use a fixed array of detectors and one or more addressable, distributed X-ray sources positioned around the patient. The distributed X-ray sources are activated at varying times and in various sequences. Because a given individual, stationary detector measures X-rays from a large number of source directions, it is difficult to use a collimator to reject scattered radiation, which leads to reduced image quality. As mentioned above with conventional CT systems, an anti-scatter grid, which allows primary photons to be detected, may accompany the detector to reduce measurement of scattered photons. In SCT designs, a scatter grid fixed to the detector is no longer possible, because every detector cell needs to have a wide acceptance angle to appropriately measure the transmitted primary radiation from a number of distributed X-ray source positions.
There is a continuing need, therefore, for improvements in CT imaging systems that can effectively reduce scatter and optimize patient dose efficiency. There is, at present, a particular need for improved system designs for SCT applications that permit measurement of data with higher fidelity.